it is observed that:
nodetool -h 127.0.0.1 -p 8080 compactionstats
pending tasks: 1
compaction type keyspace column family bytes compacted bytes total progress
Compaction SyncCoreComputedContactNetworks 119739938 0 n/a
and that is not moving (ie the bytes compacted never increase, the bytes total stay 0).
this is probably going to be difficult to reproduce, as the problem is observed when compacting 15 large sstables (total ~300G).
attaching the thread dumps (along with logs), when such happen.